Ceres (dwarf planet)

Ceres, formally designated 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. It was discovered on 1 January 1801, by Giuseppe Piazzi,[17] and for half a century it was classified as the eighth planet. It is named after Ceres, the Roman goddess of growing plants, the harvest, and motherly love.

With a diameter of about 950 km (590 mi), Ceres is by far the largest and most massive body in the asteroid belt, and contains almost a third (32%) of the belt's total mass.[18][19] Recent observations have revealed that it is spherical, unlike the irregular shapes of smaller bodies with lower gravity.[11] The Cererian surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clays.[12] Ceres appears to be differentiated into a rocky core and ice mantle,[6] and may harbour an ocean of liquid water underneath its surface.[20][21]

From the Earth, Ceres' apparent magnitude ranges from 6.7 to 9.3, and hence at its brightest it is still too dim to be seen with the naked eye.[13] On 27 September 2007, NASA launched the Dawn space probe to explore Vesta (2011–2012) and Ceres (2015).[22]

Discovery

The idea that an undiscovered planet could exist between the orbits of Mars and Jupiter was first suggested by Johann Elert Bode in 1772.[17] His considerations were based on the Titius–Bode law, a now-abandoned theory which had been proposed by Johann Daniel Titius in 1766, observing that there was a regular pattern in the semi-major axes of the known planets marred only by the large gap between Mars and Jupiter.[17][23] The pattern predicted a semi-major axis near 2.8 AU for the missing planet's orbit.[23] William Herschel's discovery of Uranus in 1781[17] near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode, and in 1800, they sent requests to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical search for the proposed planet.[17][23] The group was headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz. While they did not discover Ceres, they later found several large asteroids.[23]
Piazzi's Book "Della scoperta del nuovo pianeta Cerere Ferdinandea" outlining the discovery of Ceres

One of the astronomers selected for the search was Giuseppe Piazzi at the Academy of Palermo, Sicily. However, before receiving his invitation to join the group, Giuseppe Piazzi discovered Ceres on 1 January 1801.[24] He was searching for a star listed by Francis Wollaston as Mayer 87, because it was not in Mayer's zodiacal catalogue in the position given.[17] Instead of a star, Piazzi found a moving star-like object, which he first thought was a comet.[25] Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his observations. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Johann Bode of Berlin.[26] He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet".[17] In April, Piazzi sent his complete observations to Oriani, Johann Elert Bode, and Jérôme Lalande in Paris. The information was published in the September 1801 issue of the Monatliche Correspondenz.[25]

Soon after this, Ceres' apparent position had changed (mostly due to the Earth's orbital motion). It then appeared too close to the Sun's glare, so other astronomers could not confirm the observations of Piazzi until the end of the year. However, after such a long time it was difficult to predict its exact position. To recover Ceres, Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination.[25] He set himself the task of determining a Keplerian motion from three complete observations (time, right ascension, declination). Mathematically, this means determining a conic section in space, given one focus (the sun) and the conic's intersection with three given lines (lines of sight from the earth, which is itself moving on an ellipse, to the planet) and given the time it takes the planet to traverse the arcs determined by these lines (from which the lengths of the arcs can be calculated by Kepler's Second Law). This problem leads to an equation of the eighth degree, of which one solution, the Earth's orbit, is known. The solution sought is then separated from the remaining six based on physical conditions. In this work Gauss used comprehensive approximation methods which he created for that purpose.[27] In only a few weeks, he predicted its path and sent his results to von Zach. On 31 December 1801, von Zach and Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.[25]

The diameter of Ceres was estimated as 260 km by Herschel in 1802, and as 2,613 km by Johann Hieronymus Schröter in 1811.[28][29]

Name

Piazzi originally suggested the name Ceres Ferdinandea (Italian, Cerere Ferdinandea) for this body, after both the mythological figure Ceres (Roman goddess of plants) and King Ferdinand III of Sicily.[17][25] "Ferdinandea" was not acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in Germany.[30] In Greece, it is called Δήμητρα (Demeter), after the goddess Ceres' Greek equivalent; in English usage, Demeter is the name of an asteroid (1108 Demeter). The adjectival form of the name is Cererian,[31] or rarely Cererean,[32] derived from the Latin genitive Cereris.[33] Ceres' astronomical symbol is a sickle, (Sickle variant symbol of Ceres), similar to Venus' symbol (Astronomical symbol of Venus) which is the female gender symbol and Venus' hand mirror.[25][34] The element cerium, discovered in 1803, was named after Ceres.[35] The element palladium, also discovered in 1803, was originally also named after Ceres, but the discoverer changed its name after cerium was named. Palladium was renamed for 2 Pallas, the asteroid discovered in 1802.[36]

StatusCeres (bottom left), the Moon and the Earth, shown to scale.

The classification of Ceres has changed more than once and has been the subject of some disagreement. Johann Elert Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter, at a distance of 419 million km (2.8 AU) from the Sun.[17] Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century until further asteroids were discovered.[17][25][37]

However, as other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies.[17] In 1802 Sir William Herschel coined the term asteroid ("star-like") for such bodies,[37] writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".[38] As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering.[37]

The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification as a planet.[39][40] A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet".[41] Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.[42] However, it was not accepted, and in its place an alternate definition of "planet" came into effect as of 24 August 2006: A planet is "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit." By this definition, Ceres is not a planet because it shares its orbit with the thousands of other asteroids in the main belt and is now classified as a "dwarf planet" (along with Pluto, Makemake, Haumea and Eris). The issue of whether Ceres remains an asteroid was not addressed.[43] Dual classifications such as main-belt comets do exist, and being a dwarf planet does not preclude having other designations.[44]

Physical characteristicsSizes of the first 10 minor planets discovered profiled against Earth's Moon. Ceres is far left.
Hubble Space Telescope images of Ceres, taken in 2003/4 with a resolution of about 30 km. The nature of the bright spot is uncertain.

Ceres is the largest object in the asteroid belt, which lies between Mars and Jupiter.[12] The Kuiper belt is known to contain larger objects, including Pluto, its moon Charon, 50000 Quaoar, and 90482 Orcus, while more distant Eris, in the scattered disc, is the largest of all these bodies.[45]

The mass of Ceres has been determined by analysis of the influence it exerts on small asteroids. Results obtained by different authors are slightly different.[46] The average of the three most precise values as of 2008 is approximately 9.4 × 1020 kg.[7][46] With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2 × 1021 kg mass of the asteroids in the solar system,[47] together totalling about four percent of the mass of the Moon. Ceres' size and mass are sufficient to give it a nearly spherical shape.[6] That is, it is close to hydrostatic equilibrium. In contrast, other large asteroids such as 2 Pallas,[48] 3 Juno,[49] and in particular 10 Hygiea[50] are known to be quite irregular.

Internal structure

Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior;[6] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[6] This 100 km-thick mantle (23–28 percent of Ceres by mass; 50 percent by volume[51]) contains 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth.[52] This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling.[7][20] Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres.[7]

Alternatively, the shape and dimensions of Ceres may be explained by an interior that is porous and either partially differentiated or completely undifferentiated. The presence of a layer of rock on top of ice would be gravitationally unstable. If any of the rock deposits sank into a layer of differentiated ice, salt deposits would be formed. Such deposits have not been detected. Thus it is possible that Ceres does not contain a large ice shell, but was instead formed from low density asteroids with an aqueous component. The decay of radioactive isotopes may not have been sufficient to cause differentiation.[53]

Surface

The surface composition of Ceres is broadly similar to that of C-type asteroids.[12] However, some differences do exist. The ubiquitous features of the Cererian IR spectra are those of hydrated materials, which indicate the presence of significant amounts of water in the interior. Other possible surface constituents include iron-rich clays (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites.[12] The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids.[12] Sometimes Ceres is classified as G-type asteroid.[54]

The Cererian surface is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C) on 5 May 1991.[16] Taking into account the heliocentric distance at the time, this gives an estimated maximum of about 239 K[original research?] at perihelion.
Diagram showing a possible internal structure of Ceres

Only a few Cererian surface features have been unambiguously detected. High resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres.[54] This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation.[7][55] Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, while another was identified as the "Piazzi" feature.[7][55] More recent visible light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the nature of which are currently unknown.[11][56] One of these features corresponds to the "Piazzi" feature observed earlier.[11]

These last observations also determined that Ceres' north pole points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres' axial tilt is very small—about 3°.[6][11]

Atmosphere

There are indications that Ceres may have a weak atmosphere and water frost on the surface.[57] Surface water ice is not stable at distances less than 5 AU from the Sun,[58] so it is expected to sublime if it is exposed directly to solar radiation. Water ice can migrate from the deep layers of Ceres to the surface, but will escape in a very short time. As a result, it is difficult to detect water vaporization. Water escaping from Ceres's polar regions was possibly observed in the early 1990s but this has not been unambiguously proven. It may be possible to detect escaping water from the surroundings of a fresh impact crater or from cracks in the sub-surface layers of Ceres.[7] Ultraviolet observations by IUE spacecraft detected statistically significant amounts of the hydroxide ion near the Cererean north pole, which is a product of water vapour dissociation by the solar ultraviolet radiation.[57]

Orbit
Orbit of Ceres

Ceres follows an orbit between Mars and Jupiter, within the main asteroid belt, with a period of 4.6 Earth years. The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.08 compared to 0.09 for Mars).[4]

The diagram illustrates the orbits of Ceres (blue) and several planets (white and grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a side view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.

In the past, Ceres had been considered to be a member of an asteroid family.[59] These groupings of asteroids share similar orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres, however, was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the next-lowest-numbered family member, 1272 Gefion.[59] Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.[60]

The rotational period of Ceres (the Cererian day) is 9 hours and 4 minutes.[9]

Transits of planets from Ceres

Mercury, Venus, Earth, and Mars can all appear to cross the Sun, or transit it, from a vantage on Ceres. The most common transits are those of Mercury, which usually happens every few years, most recently in 2006 and next in 2010. The corresponding dates are 1953 and 2051 for Venus, 1814 and 2081 for Earth, and 767 and 2684 for Mars.[61]

Origin and evolution

Ceres is likely a surviving protoplanet (planetary embryo), which formed 4.57 billion years ago in the asteroid belt.[62] While the majority of inner solar system protoplanets (including all lunar- to Mars-sized bodies) either merged with other protoplanets to form terrestrial planets or were ejected from the Solar System by Jupiter,[62] Ceres is believed to have survived relatively intact.[20] (Another possible protoplanet, Vesta, is smaller; it suffered a major impact after solidifying, losing ~1% of its mass.[63]) An alternative theory proposes that Ceres formed in the Kuiper Belt and later migrated to the asteroid belt.[64]

The geological evolution of Ceres was dependent on the heat sources available during and after its formation: friction from planetesimal accretion, and decay of various radionuclides (possibly including short-lived elements like 26Al). These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation.[11][20] This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features.[20] Due to its small size, Ceres would have cooled early in its existence, causing all geological resurfacing processes to cease.[20][21] Any ice on the surface would have gradually sublimated, leaving behind various hydrated minerals like clays and carbonates.[12]

Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts.[11] The presence of significant amounts of water ice in its composition[6] raises the possibility that Ceres has or had a layer of liquid water in its interior.[20][21] This hypothetical layer is often called an ocean.[12] If such a layer of liquid water exists, it is believed to be located between the rocky core and ice mantle like that of the theorized ocean on Europa.[20] The existence of an ocean is more likely if dissolved solutes (i.e. salts), ammonia, sulfuric acid or other antifreeze compounds are dissolved in the water.[20]

Observations

When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.[13] This is generally regarded as too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet. Ceres will be at its brightest (6.73) on December 18, 2012.[14] The only other asteroids that can reach a similarly bright magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris.[65] At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky.

Some notable observational milestones for Ceres include:

* An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean on 13 November 1984.[66]
* Ultraviolet Hubble Space Telescope images with 50 km resolution taken on 25 June 1995.[54][67]
* Infrared images with 30 km resolution taken with the Keck telescope in 2002 using adaptive optics.[55]
* Visible light images with 30 km resolution (the best to date) taken using Hubble in 2003 and 2004.[11][56]

To date, no space probe has visited Ceres. Radio signals from spacecraft in orbit around and on the surface of Mars have been used to estimate the mass of Ceres from its perturbations on the motion of Mars.[47]

The unmanned Dawn Mission is currently en route to Ceres. Launched by NASA in 2007, the mission will explore the asteroid 4 Vesta in 2011 before arriving at Ceres in 2015.[22] The mission profile calls for the Dawn spacecraft to enter orbit around Ceres at an altitude of 5,900 km. The spacecraft will reduce its orbital distance to 1,300 km after five months of study, and then down to 700 km after another five months.[68] The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These instruments will be used to examine the dwarf planet's shape and elemental composition.[22]

* Movie of one Ceres rotation (processed Hubble images)
* How Gauss determined the orbit of Ceres from keplersdiscovery.com
* A simulation of the orbit of Ceres
* A website dedicated entirely to 1 Ceres
* An up-to-date summary of knowledge about Ceres, plus an Earth-Ceres size comparison (the Planetary Society)